The Federation of Master Builders has free downloadable contracts, never seen or used one, so I'm not sure as to how watertight they are legally, but they are a legit organisation so I would expect they are adequate for this type of project...http://www.fmb.org.uk/find-a-builder/free-contracts/ There's also a Consumer Guide you can download, which could be useful to read up on before you sign any contract...http://www.fmb.org.uk/find-a-builder/consumer-guides/